Not applicable.
Not applicable.
1. Field of the Invention
The present invention generally relates to protecting battery cells from various fault conditions and unequal cell performance during normal operation. More particularly, the invention relates to a cell protection circuit that provides over-current protection to battery cells.
2. Background Information
Batteries are useful for a variety of purposes, but generally must be operated in accordance with various criteria to ensure the safety and reliability of the battery and the device for which it provides power. The protection circuit described herein has been developed for use in connection with lithium batteries that are used in downhole tools. Such tools may be used for open hole logging and/or drilling purposes. Although the following background information and description of the protection circuit may be presented in the context of protecting high voltage, lithium battery packs, the protection circuit is useful for batteries used in a variety of other applications and for non-lithium battery types, particularly those that have similar safety and reliability concerns. Accordingly, the disclosure and claims which follow are not limited to the context in which the protection circuit is discussed below.
By way of definition, a xe2x80x9ccellxe2x80x9d is an individual location where chemical energy is converted into electric energy. A xe2x80x9cbatteryxe2x80x9d or xe2x80x9cbattery packxe2x80x9d is a collection of one or more cells connected in series or in parallel to produce more current, voltage or power than is provided by an individual cell.
The selection of battery type and configuration for downhole tools is influenced by various considerations. Downhole tools are typically packaged so as to have a diameter less than four inches so as to fit within a standard 8xc2xc inch diameter drill pipe. For obvious reasons, space is therefore at a premium for a downhole tool and thus battery packs should be as small as possible. Further, downhole tools usually experience relatively high temperatures presenting a potential hazard for the tool and its battery. It is not uncommon, for example, for the tool to operate at temperatures exceeding 150xc2x0 C. or even 175xc2x0 C. Also, the relatively high cost (labor and materials) of a seismic or drilling operation makes it desirable to reduce cost whenever possible. In light of these considerations, lithium cell chemistry is used in a majority of downhole tool applications today when surface power is not provided by a wireline or other means. Lithium cells, and particularly, lithium thionyl chloride (Li/SOCl2) cells provide high energy densities (i.e., a relatively large amount of energy given the size and weight of the cell when compared to other types of cells) and excellent high temperature performance. Thus, relative to many other types of cells, lithium cells last longer and operate better at higher temperatures with lower total cost.
Despite the advantages of lithium cells, they are not problem free. For instance, the discharge profile of a lithium cell must be carefully controlled to obtain the available energy from the cell and prevent hazardous conditions. Two of the most prevalent conditions that interfere with optimal Li/SOCl2 cell performance are excessive anode xe2x80x9cpassivationxe2x80x9d and cathode xe2x80x9cfreeze-over.xe2x80x9d Anode passivation refers to the formation of a layer of lithium chloride (LiCl), which is also known as solid electrolyte interphase (SEI), on the anode surface. Cathode freeze-over refers to the formation of LiCl discharge products in the outer portion of the cathode which blocks access to unused reaction sites.
A thin SEI layer is always present on the surface of the anode. This layer is formed as a result of the reaction between the lithium and the thionyl chloride electrolyte in the cell, and the layer begins to form as soon as a cell is filled with electrolyte. The LiCl generally is a desirable feature for long term storage of such cells because it helps to minimize or prevent self-discharge. It is only when the cell is placed into service that passivation becomes a problem. Anode passivation is responsible for the condition known as xe2x80x9cvoltage delay,xe2x80x9d which is the initial drop in potential observed when a load is first placed on a cell. The voltage drop is caused by the SEI layer which acts as a series resistor. As current flows through the cell, the SEI layer begins to evaporate resulting in an associated increase in cell terminal voltage. This process is called xe2x80x9cdepassivation.xe2x80x9d In a freshly manufactured cell, the drop in running potential may last for less than a second, but in a heavily passivated cell (i.e., a cell with thick SEI layer), the voltage may drop below its nominal voltage (e.g., 3 volts) for an extended period of time.
As noted above, the performance of Li/SOCl2 cells also can be detrimentally affected by cathode freeze-over which is the formation of LiCl discharge products in the outer portion of the cathode to the extent that it blocks the electrolyte""s access to unused reaction sites. The discharge of Li/SOCl2 cells results in the formation of LiCl in the cathode. If the cell is discharged at low rates (e.g., a current density of less than 2 milliamps per square centimeter), the LiCl will be evenly distributed throughout the carbon cathode, which results in efficient use of the active sites available for the reduction of SOCl2. At discharge rates greater than 2 mA/cm2, the reduction of SOCl2 occurs predominantly on the outer surfaces of the cathode. The outer surface of the cathode effectively xe2x80x9cfreezes overxe2x80x9d with LiCl, and the inner active surfaces become inaccessible. Unlike passivated anodes, which can be recovered via a depassivation process, cathodes that have been frozen over are irreparably damaged and capacity loss will result.
As noted above, passivated cells can be depassivated. This can be accomplished by placing the cells under load in a predetermined manner. Initially, the cell voltage will drop (below 3 V) due to the passivation, but gradually increase as the cell becomes unpassivated. One suitable way to depassivate a pack of cells is to place the cells under a light current load and then, as the voltage increases due to depassivation, increase the current draw on the pack.
For many cells, the voltage will begin to rise in about 15 minutes. A severely passivated cell may have a cell voltage below 3 V for a prolonged period of time (e.g., more than one hour). Any load that results in a cell voltage below 3 V for a prolonged period of time may cause cathode damage and reduce cell capacity. Batteries used for high voltage downhole tools typically are constructed from dozens or even a hundred or more series-connected lithium cells. Depassivating a pack of 100 cells might successfully depassivate most of the cells in the pack, but some cells may remain depassivated due to variations between individual cells. It is difficult to determine whether a few cells out of a hundred are passivated. Thus, as the current load is increased during the depassivation process, some cells that are still passivated will experience an increasing current demand. Because of the passivation that remains on such cells, the voltage of such cells will drop as the current load is increased. As explained below, this voltage drop can be harmful to the cell.
Cell voltage generally decreases as the current demand on the cell increases. Also, cell voltage will generally decrease as a cell ages and nears the end of its useful life. Most cell manufacturers recommend that their cells not be discharged to a point where the cell voltage is below a minimum level (e.g., 2 V). Forcing a cell below 2 V may cause bulging of the cell due to the build up of gaseous discharge products in the cell. It is also widely known that lithium cells exhibit safety concerns when the cells are discharged into reversal (the cell voltage reverses polarity). Besides cell reversal, the cells can also vent various gasses from a short circuit at elevated temperatures. Manufacturers also warn that excessive loading of 3 or more cells in series has been known to result in venting caused by cell reversal.
Accordingly, because current is increased during the depassivation process and the voltage of passivated cells decreases with increases in current, it is possible to over drive a passivated cell during the depassivation process. This is particularly problematic for large strings of cells in a battery in which it is difficult to detect a few passivated cells out of numerous other depassivated cells in the battery. Over driving a passivated cell in this manner may cause such problems as cell reversal and off gassing. A solution to this problem is needed.
A similar concern is also present during the normal use of a battery. The battery provides the current needed by the device (e.g., downhole tool). If one or more of the cells in the battery are passivated, the cell voltage for such cells may drop to a dangerous level. Also, a cell that is not passivated, but is nearing the end of its useful life, may be unable to provide the necessary current at an acceptable voltage level. As such, the voltage of such a nearly spent cell may be forced to a dangerously low level at the current level demanded by the load.
In summary, there are various reasons why a cell may be forced to an undesirable or dangerously low voltage level. Several of such reasons are given above. Regardless of the reason, a way to guard against such a condition is needed.
The problems noted above are solved in large part by a protection circuit which couples to and protects a cell. The protection circuit generally limits the current that can flow through the cell when the voltage across the cell falls to a predetermined minimum threshold. By limiting the current through the cell in this situation, the voltage on the cell will not fall below the minimum safe level.
In accordance with one embodiment of the invention, the protection circuit includes a transistor coupled in series with the cell and a bypass device coupled to both the transistor and the cell and in parallel with the transistor and cell. The transistor preferably comprises a metal oxide semiconductor field effect transistor (xe2x80x9cMOSFETxe2x80x9d) and more preferably either an n-channel, enhancement mode MOSFET or a p-channel, enhancement mode MOSFET. The bypass device preferably comprises a diode that permits current to conduct around the cell being protected when the transistor limits the current through the cell.
If desired, a delay element can be included in the protection circuit to slow the change in voltage across the transistor with short duration pulse loading. The delay element may comprise a resistor coupled to a capacitor to provide a desired R-C time constant. The delay element increases the performance of the cell by not activating the current limiting function of the transistor.
In a battery comprising a plurality of cells, each cell can have its own protection circuit, thereby providing improved protection in a multi-cell battery. The protection circuit, in fact, can be made in the form of a disk or wafer that physically is disposed between adjacent cells in a string of serially connected cells. It is also possible to include the protection circuit within each cell. In this way, the cell""s voltage can be maintained at a safe level. These and other advantages will become apparent upon reviewing the following disclosure.